The present invention relates generally to a method for manufacturing dental restorations such as dental crowns and dental bridges and more specifically to a method of manufacturing dental restorations using high strength ceramic materials.
Strength and reliability are important factors to consider when manufacturing dental restorations. Dental restorations must be able to withstand the normal mastication forces and stresses that exist within an oral environment. Different stresses are observed during mastication of different types of food, which can be experimentally measured by placing, for example, a strain gauge in inlays on the tooth. Stresses differ depending not only on the type of food, but also on the individual. For example, stress values may range from 570 to 2300 lb/inch2 for a single chewing thrust on a piece of meat and from 950 to 2400 lb/inch2 for a single thrust on a biscuit. The physical properties of dental restorations must be adequate to withstand the stresses applied by the repetitive forces of mastication.
Ceramic materials have proven to be reliable in the fabrication of single unit dental restorations. U.S. Pat. No. 4,798,536 to Katz, U.S. Pat. No. 5,653,791 to Panzera et al., and an article by Kabbert and Knode entitled xe2x80x9cInceram: Testing a New Ceramic Materialxe2x80x9d, Vol.4, pp 87-97 (1993) each disclose ceramic compositions having leucite therein to provide strength and reliability to dental restorations. The strength of the materials is in the area of 170 MPa which is much higher than that of conventional porcelain which exhibits strengths of about 70 MPa. Nevertheless, the strength and/or toughness values of the aforementioned ceramic materials may not be adequate for the fabrication of multiple unit restorations.
Due to the unique shape and size of dental restorations, it is often difficult to achieve uniform physical properties which are effective in dental restorations. Moreover, fabrication techniques often involve manual blending of powders, liquids and dispersants which may not provide the optimal homogeneity for the mixture. This may result in uneven shrinkage and poor mechanical properties in the finally sintered restoration. The standard deviation of mechanical properties is very high for manually blended mixtures and the products produced therefrom are inconsistent in properties. Furthermore, nonuniform particle size may lead to porosity that will result in weak structures.
U.S. Pat. Nos. 4,265,669, 4,585,417 and 5,975,905 are directed to the fabrication of dental restorations using ceramic powders in combination with binders and are hereby incorporated by reference. U.S. Pat. No. 4,265,669 to Starling, et al. disclose dry flowable ceramic powder mixtures containing about 10-15% of a silicone resin. The mixtures are shaped and sized as desired and sintered to monolithic structures without any shrinkage or distortion occurring during firing. The mixture is required to be soft at about 30xc2x0 C. and hardens at about 150xc2x0 C. As a result, a complicated molding transfer process must be performed. Additionally, the firing process for the structures is lengthy, typically as long as ten hours or more which reduces the chances for an efficient and facile operation. Similarly, U.S. Pat. No. 4,585,417 to Sozio, et al. involves a complicated process. The firing time required for the process is very long, requiring a minimum of two stages over a period of twelve hours, which is impractical for many dental laboratories.
There is a need to provide high strength, ceramic restorations having structural integrity and reliability and optimum bonding properties. It is desirable to produce high strength ceramic restorations that are compatible with a wide range of cost-effective ceramic materials. It is beneficial to provide dental restorations having uniform physical properties throughout the restoration and to simplify the process of manufacturing dental restorations to provide a more user-friendly process.
These and other objects and advantages are accomplished by the process herein comprising applying one or more strips of ceramic tape or ribbon to a mold for a dental restoration. The ceramic tape is in its green state so that it is malleable and formable to the mold, but does not break or crack as it is applied to the mold. Pressure may be applied to further form or adapt the ceramic tape to the shape of the mold. Heat is applied simultaneously with pressure or in a separate step to achieve high density and strength in the ceramic material. A vacuum atmosphere may be used with the application of pressure and/or heat. One or more layers of surface material such as porcelain may be applied to the ceramic to form the dental restoration. The process is useful in the manufacture of dental materials or restorations including but not limited to orthodontic appliances, bridges, space maintainers, tooth replacement appliances, splints, crowns, partial crowns, dentures, posts, teeth, jackets, inlays, onlays, facing, veneers, facets, implants, abutments, cylinders, and connectors.
In another embodiment herein, ceramic powder and one or more media materials are combined to form a homogeneous mixture. The mixture may then be used to form a dental restoration as is or may be used to form feedstock such as filaments or wires which are then used to fabricate a dental restoration. The filaments or wires may be used in a fused deposition-modeling machine to build dental restorative materials by computer aided design (CAD) software. The restorative materials include but are not limited to an orthodontic appliance, bridge, space maintainer, tooth replacement appliance, splint, crown, partial crown, denture, post, tooth, jacket, inlay, onlay, facing, veneer, facet, implant, abutment, cylinder, and connector.